Speaking of Psychology - What studying twins can teach us about ourselves, with Nancy Segal, PhD

Episode Date: February 24, 2021

From movie plots to ad campaigns to viral videos, if they feature twins, they grab our attention every time. But it’s not only the general public who are fascinated with twins. Over many decades, twins have garnered attention from psychologists and other researchers because of what they can tell us about how our genes and environment interact to make us who we are. Nancy Segal, PhD, a professor of psychology and director of the Twin Studies Center at California State University, Fullerton, talks about the state of twin research today and what we've learned about twins, and from them, over the decades. Are you enjoying Speaking of Psychology? We’d love to know what you think of the podcast, what you would change about it, and what you’d like to hear more of. Please take our listener survey at www.apa.org/podcastsurvey. Links Nancy Segal, PhD Twin Studies Center at California State University, Fullerton Music Funny music (orchestra) by Darkash28 via Freesound.org     Learn more about your ad choices. Visit megaphone.fm/adchoices

Twins have long captured the public imagination. Movie plots, ad campaigns, viral videos of twin toddlers babbling at each other in what seems like a secret language. Something about twins grabs our attention every time. But it's not only the general public that's fascinated with twins. Twins have also garnered their share of attention from psychologists and other researchers because of what they can tell us about how our genes and environment interact to make us who we are. Where is twin research today? As our understanding of human genetics has advanced in recent years, do twin studies remain vital? What about twins who are raised apart and meet later in life?

What can we learn from those natural experiments? And why are so many people, scientists, and lay people alike, so fascinated with twins? Welcome to Speaking of Psychology, the flagship podcast of the American Psychological Association that examines the links between psychological science and everyday life. I'm Kim Mill. If you enjoy speaking of psychology, the conversation doesn't have to stop when the podcast is over. Get unlimited access to hundreds of videos about the latest in psychology for just one low price by registering for access to APA's 2020 virtual convention. Start with some of our selected videos featuring psychology's biggest names, discussing such issues as COVID-19, racism, and stigma.

Or let your curiosity take over and use our on-demand library to explore any time. topic. Go to convention.apa.org slash podcast. That's convention.a.a.org slash podcast. Our guest today is Dr. Nancy Siegel, a professor of psychology and director of the Twin Studies Center at California State University Fullerton. She has spent her entire career studying twins, beginning with her work on the Minnesota study of twins reared apart in the 1980s and 90s. Today she continues to study identical and fraternal twins, twins reared apart, and other unusual pairings, as she calls them, even unrelated people who look like twins. She's written six books about twins and twin research and is working on a seventh.

She is also, as it happens, a twin herself. Welcome to speaking of psychology, Dr. Siegel. Thank you so much for having me. Let's start with a broad question. Twin studies have a long history in psychology. Why is that and what have we learned over the years? Well, twin studies are fascinating and it's a very simple and very simple and very very elegant design. One simply compares the resemblance of identical twins who share all their

genes to the resemblance of fraternal twins who share half the genes on average. For most measured traits, we find that identical twins are more alike than fraternal twins, which is consistent with the genetic influence on that behavior. And there are many variations on what I just described, which is the classic twin method. But as you said in your introduction, we can study twins we can study twins living apart for long periods of time. And I have also been studying some very unusual pairings, which I hope we have time to get into. I'm sure we will. So you've been studying twins since the 1980s. How have the questions that you and other researchers ask changed over time as our understanding of genetics has become more sophisticated? For example,

a recent article in the Atlantic talked about how twin studies are advancing our understanding of epigenetics or how genes are expressed. So I'm wondering if you can explain what that means. Yes, sure. But let me just back up for a second. You wanted to talk about how questions and interests and focus has changed in twin studies over the years. So when I began in the 1980s, this was just when behavioral genetics was being admitted into this mainstream of psychology. And so investigators were interested in establishing a genetic basis for human behavior. And twin studies and adoption studies were a very effective way to go. Then things began to change and people began looking to see if they could find the actual genes that were linked to behaviors.

And that's partly where epigenetics comes in. Now epigenetics refers to the turning on and turning off of genes. What environmental triggers, either before birth or sometime after birth, will activate a gene or perhaps silence it. And this is where identical twins who differ in fundamental ways, maybe of greatest use to us in the medical sciences, because we know that the similarity rate for identical twins are diabetes and multiple sclerosis is only 50%. Schizophrenia, maybe 40%. So if they come into the world with the same genes, why is it that one twin expresses it and the other one does not? This is information that we can all use to assist individuals who are affected in the non-twin population. And epigenetics came into play when Scott Kelly

went into space, for example, right? He's an identical twin, and his brother was on Earth. And when Scott Kelly came back, he had gone through some genetic changes. Can you explain that? Yes, he did. The flight travel that Scott Kelly took had him up at the International Space Station for almost a year while his twin brother Mark remained on Earth. And prior to the trip, they both had completed a very comprehensive set of tests, intellectual, physical, genetic, things of that sort. And they think, found that contrary to expectation, it seemed that Scott Kelly's chromosomes actually reflected greater longevity. One would expect that they should have not shown that. But most of his epigenetic changes have reverted back to what they were normally. But that is a fascinating

study because we will never again get two identical twins who are both astronauts and one willing to take the trip. That's amazing. And is that common with epigenetics that things revert? It's trait specific. It's trade specific. One wouldn't really know that. Yeah. Identical twins are not exact copies of each other in the sense that you've studied some identical twins who show a fair amount of physical variation, such as differences in height or hair color. What are the causes of these variations? Well, the causes of variation are many. If you talk about height and weight, it probably has to do with prenatal nutrition that might have been unequally distributed between the two twins. If identical twins share their blood systems, what we call fetal transfusion

in the womb, that can also create differences in size and vigor, things like that sort. And if really to a severe degree, it might even lead to the demise of one of the twins. And then I mentioned the epigenetics before. If twins somehow get exposed to different environmental events, whether global or micro environmental events, this can also trigger something in one twin and on the other. Genetically speaking, too, identical twins may not be exactly alike. Sometimes there are errors in copying when cells replicate, and so there could be spontaneous mutations in one twin that the other twin does not have. So identical twins are less alike than people think they are. They're more alike than any other pair of people, but they are less alike than people think they are. And that's what makes

I'm so fascinating. Now, I've seen in some interviews you've corrected people who have used the term nature versus nurture. Why do you think that term is inaccurate? It's inaccurate because it does not appreciate the interplay between nature and nurture. So we prefer to say nature dash nurture or nature via nurture or something of that sort. We try to disentangle these effects in populations. So when we say that intelligence has a 75% genetic effect or height is 95%. We're not talking about a single individual. We're talking about the variation in a population. In a single person, you cannot separate out the two

because genes and environments are inextricably intertwined. You're a fraternal twin yourself. How has that experience as a twin influenced your research? Well, being a fraternal twin, I was always fascinated with being a twin, And although I never got the adulation that identical twins get when people meet them. But I knew from a very early age that there had to be something fundamentally different about us because we shared so many experiences in common and yet we were so different in appearance and our interest,

and tastes and things of that sort. So I would say that it certainly propelled me into the field. How it's influenced me? Not really, because I let the data drive that. And I've been surprised sometimes by certain things and certain things I've expected have come true. but you have to keep an open mind. But I will say that being a twin has been a great advantage because in soliciting subjects for my projects,

the minute they discover that I'm a twin, it creates this bond that you just understand what's going on, that we share something fundamentally important. And that's been a real advantage. There are a lot of myths and legends out there about twins that they've got some kind of a telepathic connection, that they can share secret language. And I know one of your books is called twin misconceptions,

false beliefs, fables, and facts about twins. What are some of the common myths that you address? And do any of these have grains of truth? Well, the ones that I address mostly are, I mean, there's so many, there must be out 70 that I deal with in the book. But some people think that fraternal twinning skips generations. Now, is there a kernel of truth? It could do that in some families, but there's no actual rule for that.

We know the fraternal twinning runs in families, but the actual mode of transmission has not been identified. Some people feel that identical twins should always be separated in school when they enter. And I'm really against that. I think you have to take every pair on a case-by-case basis. We don't have firm policies regarding singleton children, and I think that this treats twins unfairly. Other myths that identical twinning does not run in families. Now, we thought that for a very long time. And yet some recent research from Sweden and from Singapore looking at inbred populations has found that there are these pockets of people around the world in India, in Iran, where there are multi-generations of just identical twins. And so we think that within

some families, there's this tendency toward zygotic splitting or splitting of the single fertilized egg. In my study, well, it wasn't for a study, but it was for my book Indivisible by two. It was kind of a, it's an interesting book. It's kind of like Oliver Sacks, the man most took his wife for a hat book, but for twins, where I bring in the human interest and the science. And one of the couples that I studied were actually a set of identical male twins married to a set of identical female twins. They live very close by. And one of the couples had identical twins. And so that's twins having twins. And in my book, Twin Mithconceptions, I have a photograph of identical twin women, each of whom had a pair of identical twins. So it seems that there is some

factor in some families that gives you this genetic tendency toward that kind of twin. And that creates some kind of a weird relationship, right? Because the twins are, they're genetically the same. I mean, so is your uncle? You're also. Well, yes. Maybe you can explain this because I'm not doing a good job. Yeah. Yeah. Yeah. What happens is that if you have identical twins who marry identical twins, the offspring are legal first cousins, but they're also genetic full siblings. because every parent is genetically interchangeable. That's somewhat rare, but a much more common scenario is identical twins who just marry unrelated people. And their children would be genetic half-siblings, in addition to being cousins.

But I will tell you one story that I have also in one of my books, I think it was an entwined lies, my first one. I have a set of identical twins who married identical twins, and they had children on the same day, which makes them genetically equivalent to fraternal twins. But see, they're enormously informative from the point of view of research because you've got people who are, you know, you can look at the relation between, say, a twin aunt and a twin mother towards children. And they share the same genes, but one is raised with one and one is not raised with the other. So you can look at all sorts of stuff.

Wow, that's wild. I can't imagine doing that genealogy. Yeah. So you address the question about twins, whether they should be in the same class, during their education, right? That's something that you've looked at. I'm wondering, when I grew up, there were twins in my school and parents always dressed their kids the same. Is there anything potentially damaging to children who don't get their identity, their own identity because they're always dressed like their twin?

Yeah, well, I think that can be a problem. And so I always encourage parents to dress the kids somewhat differently, maybe in the same outfits with contrast and colors, to have a name badge, cut the hair a bit differently, so that people will call them by their name and not by twin or hey you. I think, though, that twins should be allowed to enjoy their similarities at times, if they want to dress up alike, an occasion, nothing wrong with that. But you don't want them to rely on that to get attention from other people. The twin birth rate has more than doubled from 1980 to 2007, although it declined a little bit since then. What's caused the shifts in the rate of twinning and have you seen any corresponding interest in your work as a result of that over the years?

Well, yeah. So in 1980, the twinning rate was about 1 in 60 babies. And later on, 20 years later, 30 years later, it was one in 33. And the reason that's gone up so much is because women are delaying the childbearing years, which creates a greater tendency for fraternal twinning. After all, for older mothers, you know, mistakes happen. And we are optimally. designed to release one egg at a time. So two eggs at a time is kind of this mistake, although kind of a pleasant mistake, but it's still something's not supposed to happen. So that's one reason. Another is that we're better able to manage twin pregnancies now so we can detect them and manage them better. And then within vitro fertilization, if women delay childbearing for too many years

and they're unable to conceive, then they sometimes go the root of in vitro fertilization where eggs and sperm are united outside the womb and then later implanted. And that also has increased not just the fraternal twinning rate, but also the identical twinning rate, although not as dramatically. Now, the great rise in twinning, of course, has focused attention on twins so much. We see them in literature. We see them in the news, TV commercials all the time. People are fascinated with it.

And I think that leads to another interesting idea. Why? Why are people fascinated? So I began asking people and they'd say, well, they just are. They're fascinating. But I could not get an answer. So I've thought a lot about it. And I think it's because we all are raised to appreciate individual differences in appearance and in behavior.

So when we encounter two people who look and act so much alike, it challenges our beliefs and how the world works. And some people find it, well, everyone finds it intriguing. Some people find it a little upsetting to see that much closeness. But I think that's the clue. And with twins raised apart, you know, we have such a strong reverence for family. The idea of these two very close people being raised apart just does not sit well. And when we hear about the reunions, they're just so heartwarming. I have a whole series of tapes of twins meeting for the first time, and I've seen it so many times.

And every time I tear up, it's just that moment of me, that out-of-control glee is just amazing. That's got to be a lot of fun. You go through life, thinking that it's just you, and then you discover. It's you and me. There's another one like me. Yeah. Great. So let's talk a little bit about the research projects you have going on now.

You're continuing to study twins reared apart, right? I am. I have a study ongoing that looks at young Chinese twins who've been separated at birth indirectly through the one-child policy. And I get them. It's the only prospective study in the world. Most reared-to-part twins studies have found twins as adults. and had to reconstruct their childhood events.

But I'm getting the twins over time. Every pair has now had about two assessments, although I'm still completing the second one for some of the pairs. And so the twins live with different families in the U.S., sometimes in Canada or other places around the world. And so I'm able to get the twins tested in their local areas and parents fill out questionnaires. Now the kids are growing up so I can do a little more with them.

And I've studied 23 pairs of them so far, 15 identicals and eight fraternals. And I just hope I can continue because grant money is scarce. But I would just love to find some more. I really would. Now, I also do case studies on adult twins who find each other. And I've got one right now. These twins from South Korea just met each other.

Just in the last week or so, and I'm so excited. And so I have a colleague in South Korea who will test the South Korean twin, and I'm going to test the twin who lives here. And they didn't know about the other? No, they didn't know about the other until, let's see, well, the one in South Korea knew, because what happened there was that she was out with, well, the mother had twins. She obviously knew that. The twins were maybe two or three, and the grandmother took one twin out on a walk or something

and lost her in a market, never found her again. And the twin ended up back in a orphanage and was adopted. But she never knew she was a twin until she did, it's a program now for Korea and adopties so they can submit their DNA and see if it matches. And that's how she found her family. I'm also fascinated by a study you did a few years ago on doppelgangers, the people who are completely unrelated, but they look uncannily alike. Why were you interested in studying this phenomenon and what did you learn? Yeah, that's a great question. See, there was a criticism of twin research held by some that identical twins are alike in personality because people treat them a

certain way based on their looks. And that never made sense to me. It just never made sense to me. I think what happens is that people respond similarly to identical twins, but because they evoke certain reactions from these people. So the way to test that I realized was to use unrelated people who shared no genes, but shared their looks. And so I reasoned that if they were as a like as identical twins raised apart, well, then maybe I was wrong. But if they were very different, then it would mean that their genetics were so different that they, you know, they just did not, they were not going to be similar in personality. And so I worked with somebody in Canada who, a photographer, Francois Brunel, who takes photographs of these people.

He's a photographer. I think he was a doppelganger himself. And so I got some of my people from him. And then a British television show gave me eight pairs. And I find them occasionally, you know, here and there, I find. them and they're just amazing. And so I published two or three papers on these cases, and I find that these doppelgangers who were raised apart, but look a leftly like, are so different than personality. And when they meet, they don't feel a certain kinship. The identical twins feel it almost instantly.

These people are sort of curious and maybe it lasts for a minute, but it doesn't last. And do they actually see the resemblance? They always get it? Sometimes and sometimes not. You know, this is another interesting thing that sometimes Sometimes if you talk to identical twins, they don't think they look alike. It's a very, very funny thing. So how do you find participants in your studies? Is the Internet making it easier? Oh, absolutely it is.

And, you know, I really have never solicited people. People come to me. I feel like Sir Francis Galton back in the 1870s, when people would say, I want to be in your studies. I mean, I'm just getting people all the time. It's just amazing. I'm lucky that I have an international network. people send me emails and contacts all the time.

I found one of my most interesting pairs, well, actually the situation in Colombia, South America, involving two sets of identical male twins who'd been accidentally switched at birth. So it was an exchange of one twin from each pair. And they grew up in very different environments. And when I learned about this through a colleague, I went right down to Bogota to study them. And it was a book came out of that called Accidental Brothers, published in 2018. But let me get back a minute to the studies of twins raised apart. I think it's also important to recognize that twin studies have had a wonderful history,

but there have been a couple of blemishes along the way. And the book you mentioned earlier that I'm writing right now, my seventh, it's actually all written in the hands of the editor, so I'm a free person. But that one is going to be called deliberately divided inside the controversial study of twins triplets adopted apart. They'll be out in the spring of 2021. And I think that most of your viewers have probably seen or at least heard about the documentary film Three Identical Strangers that talked about identical triplets who were separated and in a secret study. So I went back and I researched that study, the history, the origins, the implications, all the people and the legal and ethical

implications. And so it's a pretty big book, but it had to be written. It had to preserve all of that. and it was a tribute to the twins who were separated at birth, who were separated because a researcher or a psychiatrist felt it was better for them. I mean, it just, there was no literature to support that whatsoever. And yeah, tell us a little bit more about this psychiatrist. His name was Peter Newbauer, right? No, no, no. Well, there were two psychiatrists.

Oh, oh. Yeah. Dr. Viola Bernard was the, she was the psychiatrist that had been hired by the Louise Weywey agency and just to give advice on adoptions and things. And she felt that identical twins or even any twins should be raised at parts they'd develop their own identities and parents would not be overburdened. But there was no literature to support that. Absolutely none. But that went on nevertheless. And then this other psychiatrist, Peter Newbauer, discovered this and he decided to study the twins. It was a collaboration. And it's really hard to know which came first,

the study, the theory. I mean, it's difficult to know exactly. And I talk all about that in the book. And so the twins were studied from the time of birth until 12 years. And of course, the data are locked up at Yale and not to be sealed until 2065. Yeah. That's another issue I deal with. And some of Dr. Bernard's materials at Columbia have been sealed too. But there's enough available there that I was able to really get a lot of material. I look forward to seeing that. So what are the remaining unanswered questions in the field? What do we still don't know?

I'm sure there are many, many, but maybe you can give us a few. Well, you know, one very vexing question that's still never been answered satisfactorily is why does the fertilized egg divide? What causes a divide? There are theories about it, but we really do not know. We really do not know. And it used to be thought that the timing of the split from conception until day 14 had to do with the number of placenta and the number of fuel membranes. But even that is in dispute.

So it's really, really quite fascinating. And anything else that's on the horizon that you want to tell our listeners about any other work that's percolating? I think we're still going to continue with looking at how epigenetics is going to shape our understanding of human behavior and disease, that's certainly going to take off. But I think that it's always so important to stay very close to twins just to get a sense of what they're like, because just being with them gives you so many great hypotheses about how things could happen. In Minnesota, for example, we used to see that identical twins had a lot of quirks in common. We had a pair who used to read books from back to front, and he used to wash his hands before and after using the

bathroom, all these kinds of things. And so when you see it in one person, you figure, well, it's just a habit. But with two people who do it, who are reared apart, you figure there have to be something more. And so I think one of the projects I may attempt in the fall, I'm sorry, in the spring when I teach my graduate seminar on twin studies, is I'm going to have my students go back to the older literature and document all of these unusual behaviors in the identical twins and some other ones. And we'll just see, you know, what are they? Can they? Can they? I'd be studied systematically. How do we explain them?

Well, Dr. Siegel, this has been really fascinating. I think the research that you do is quite amazing. And I really appreciate your taking the time to talk to me today. It's my pleasure. You can find previous episodes of Speaking of Psychology on our website at www.w.spinkingof Psychology.org or wherever you get your podcasts. If you have comments or ideas for future podcasts, email us at speaking of psychology at APA.org.

That's speaking of psychology, all one word, at APA.org. Speaking of psychology is produced by Lee Weinerman. Our sound editor is Chris Kondyen. Thank you for listening. For the American Psychological Association, I'm Kim Mills.